Active Fish Capture Methods Chapter 7 Active Fish Capture Methods

7.1 Introduction Moving gear/nets through water Collecting Fish Crustaceans & Other Inverts.

3 Main Gear Types Towed Nets Dredges Surrounding Nets Plus Others (Hook and line, cast nets)

Standardization of effort Pull trawl fixed time Sweeping specific area

Requirements Larger boats More manpower Less sampling time than passive gear

7.2 Net Material and Construction Natural Materials - cotton, hemp, linen Thick, heavy Rotting is a problem Synthetic materials - polyethylene, polypropylene Stronger, thinner Less prone to decay

Mesh size (cont.) small fish pass through mesh, measure consistently

Mesh size Bar length - distance knot to knot along diagonal Stretch measure - knot to knot distance when mesh is stretched

Hanging ratio (E) E = rope length / stretched length of netting Or use hanging % = 100 * ( 1 - E ) Range for trawls (E = 0.6 - 0.8)

7.3 Dragged or Towed Gears: Trawls Funnel-shaped with cod-end (narrow backend) Midwater or bottom Beam or otter trawls

Beam Trawls Fixed width Sweep fixed area consistently Somewhat cumbersome if beam is large

Otter Trawls Otter boards or trawl doors use force of water Oval or square, reinforced skids Boards hold open net Mouth width depends on force (inconsistent)

Trawling advantages Fish in good condition (unless deep trawls... pressure changes) For release of live specimens, short trawls (5-15 min) Quantitative index of pop abundance

Trawl disadvantages Can't sample when bottom is irregular (stobs, rocks) Need powerful boat (40 hp or greater) By-catch of other species?

Examples of Sampling Programs MARMAP, SEAMAP - National Marine Fisheries Service Eastern Seaboard Yankee 36 trawl (18 m headrope, 24 m footrope) Transects perpendicular to shore out to cont. shelf

Examples of Sampling Programs Great Lakes Fisheries Survey - Great Lakes Laboratory Bottom & midwater trawls & acoustic surveys 5 - 150 m depth seasonal variation

Bottom Trawl Modifications Rollers on the sweep chain Tickler chains on the sweep chain Plastic strips on the bag to prevent snagging Size & material of doors

Midwater Trawls (cont.) Four seams Mesh at mouth coarse, mesh finer toward cod-end

Midwater Trawls Depth determined by boat speed and warp out Determined by angle or by pressure sensor on net Midwater Trawls

Use of Midwater Trawls Sample pelagic fish Ground truthing for acoustic surveys Sampling larvae and juveniles (1 mm mesh)

Examples Remote Midwater Trawl Isaacs-Kid Midwater Trawl MOCHNESS BIONESS

Evaluating Gear Performance Did net catch fish? Net hang-up on bottom? Cod end tied? Crossed or twisted trawl doors?

Technology to Evaluate Gear Depth/Pressure sensors Laser distance measures Video camera mounted on gear

7.4 Dragged or Towed Gears - Dredges Heavy metal frames Chain link bags Cutting bars or teeth dig into substrate

Scallop dredge Rectangular metal opening Triangular frame attached to single warp Bag of metal rings lined with smaller mesh net Pressure plate to force dredge to dig into substrate Oyster dredge is higher, with a shorter bag

Monitoring performance Temperature/depth sensors Check the bottom of the dredge, abrasion will shine up the metal Debris (rocks, wood) in the dredge usually means catch will be low

Examples of dredge surveys (cont.) 1. NMFS sea scallop survey Along East Coast 2.44-m wide 5.1 cm diameter rings 3.8 cm polypropylene mesh liner

Examples of dredge surveys 2. DFO survey of Georges Bank

7.5 Surrounding or Encircling Gear beach seines, purse seines, lampara net trap fish inside fence of mesh area sampled is fairly standard

Seine components (cont) Float line - cork, styrofoam, or plastic floats hold mesh upright Lead line - lead weights attached or lead in core of polypropylene line

Seine components Bunt - section of mesh wall where fish are concentrated Bag - small pocket sewn into the bunt for further fish concentration Mesh - forms the wall of the seine.

Beach or haul seine long regular wall of mesh with bunt (& maybe bag) walk wings around fish retrieve, but ensure lead line (mudline) stays on bottom

Fishing a beach seine (cont.) fished near shore by wading; no obstructions to lift lead line set in semi-circle; retreive both ends or set perpendicular to shore, walk along; then offshore fisher curls to shore

Fishing a beach seine 1-3 wings or leaders (guide fish) enclosure with throat float anchor pay attention to capture efficiencies - vary diel, seasonal, by species

Purse seines For pelagic (open water) species

Purse seines Or demersal if leadline goes to the bottom Can fish with one or two boats

Fishing a purse seine (cont.) Wall of mesh encircles fish Pull purse line from one or both ends Bottom of net cinches shut like drawstring purse

Fishing a purse seine Fish are in a bowl of mesh Bowl is made smaller until fish in bunt of seine

Examples 1. Juvenile coho salmon - Oregon and Washington 495-m long seine set in transects up & down coast catch showed juveniles migrate north in ocean

Examples (cont.) 2. Rainbow trout - Lake Washington 600-m long, 37-m deep, 25mm stretch mesh netting collect fish for food habit study

Lampara net For catching fish near surface Used over rough bottom where beach or purse seine won't work Leadline shorter than float line After circling fish, ends of leadline pulled Leadlines come together making a bowl full of fish

7.6 Other Active Sampling Gears (cont.) Push nets Rectangular rigid frame with mesh behind Pushed in front of small boats - sample fish fry

7.6 Other Active Sampling Gears (cont.) Neuston nets push net towed to the side or behind a boat

7.6 Other Active Sampling Gears (cont.) Lift nets three line bridle on a bowl of mesh bait the mesh or attract fish over net with light lift the bowl and trap the fish (or crabs)

7.6 Other Active Sampling Gears (cont.) Pop nets Rectangular frame of mesh Set on bottom Released to pop up and form a box

7.6 Other Active Sampling Gears (cont.) Dip nets Circular net on a pole Lift fish from water - during electrofishing Remove fish from containers

Other Active Sampling Gears (cont.) Fish Wheel Ferris wheel for fish Native Americans harvest anadromus fish this way

Other Active Sampling Gears (cont.) Cast nets (requires skill) Circle of mesh Weighted edges Draw-string for cinching net closed Usually near-shore for bait fish

Other Active Sampling Gears (cont.) Drop nets Rigid cylinder or box of mesh (usually <1m^2) Thrown or dropped in sample area Fish removed from fixed area… quantitative sample

Other Active Sampling Gears (cont.) Angling Rod and reel sampling To collect brood stock To collect fish in good shape for radio telemetry studies When other gears won't work

Other Active Sampling Gear Spears Trident Spear with barb Usually clear water - tropical reef fish Hawaiian sling or speargun

7.7 Gear selection Why do you need the fish? Relative abundance or density estimate - trawl Live specimens for study - short trawl Tissue requirements or diet studies - seine, spear, hook and line

What is the environment like? Shallow - beach seine Open water - purse seine Smooth bottom - otter trawl Rough bottom - lampara net, scallop dredge

What is life history of fish? Demersal - otter trawl Pelagic - purse seine Associate with structure - hook and line Littoral zone - beach seine

Gear selectivity Large, fast swimmers (tuna) outswim active gears Small fish pass through coarse mesh of trawls Ontogenetic changes in habitat affect selectivity Juveniles inshore (beach seine) Adults offshore (otter trawl)

7.8 Sampling Problems (cont.) Structure Impedes progress of gear - woody debris in rivers Gear won't go into habitat - elkhorn or staghorn corals

7.8 Sampling Problems Lake size Small deep lake needs big boat to set trawl deep But small lake won't fit